The present invention generally relates to transfer cases for providing xe2x80x9con demandxe2x80x9d torque to the front or rear wheels of a four wheel drive vehicle. In particular, the present invention relates to a planetary gear set coupled with a torque sink device for transferring torque to the front or rear wheels of a vehicle driveline during slip events.
Computer controlled torque transfer systems for all wheel drive and four wheel drive vehicles are well known in the art. These torque transfer systems provide traction control and enhanced vehicle stability by actively controlling torque distribution from front to rear, side to side, or a combination of both using computer controlled torque biasing devices. Typically, in all wheel drive or four wheel drive vehicles having xe2x80x9con demandxe2x80x9d torque transfer systems, torque is distributed between front and rear wheel pairs using a transfer case. In these xe2x80x9con demandxe2x80x9d systems, one set of wheels is directly driven by a power source, or engine, via a transmission, drive shaft and axle. The other set of wheels receives torque from the power source through a transfer case using a coupling mechanism such as a center differential or active clutch. These coupling devices regulate power proportion between the front and rear wheels. In the xe2x80x9con demandxe2x80x9d torque transfer system, a first set of wheels is primarily driven by the power source and a second set of wheels is driven only when events occur that require torque transfer, such as, wheel slippage, understeer correction or oversteer correction. Where the primary drive wheels are the rear wheels, torque is transferred to the front wheels only when, for example, the rear wheels begin to lose traction. By transferring torque to the front wheels, the front wheels can drive the vehicle until the rear wheels regain traction.
Current systems regulate torque using a clutch system that acts directly between a drive and driven shaft. These clutch systems possess a large number of moving parts and require relatively large actuation forces to transfer torque and control wheel slippage. The clutch based systems also generate large amounts of heat during slip events and require extensive development of the friction material and lubricant in order to avoid xe2x80x9cstick-slipxe2x80x9d phenomena and clutch moan and chatter. In order to achieve large torque values, many friction surfaces are required. The friction surfaces are prone to wear and therefore may experience wear or degraded performance over the life of the product.
The transfer case of the present invention distributes torque through a vehicle driveline using a planetary gear set coupled with a torque sink device. The planetary gear set provides a compact torque distribution system and, due to gear reductions through the planetary gear set, requires a relatively small braking force to distribute torque.
In accordance with one aspect of the present invention, a transfer case system is provided having a planetary gear set. The planetary gear set is made up of a sun gear, a ring gear, at least one planetary gear intermeshed with the sun gear and ring gear, and a planetary carrier rotatably connected with the at least one planetary gear. The transfer case system also comprises a torque sink connected with the sun gear. The transfer case system further comprises a torque input shaft. The torque input shaft is operably connected with the planetary carrier and is also coupled with a primary drive train. When the torque sink applies a braking force to the rotation of the sun gear in response to instructions from a controller, the ring gear outputs torque to a secondary drive train.
In accordance with another aspect of the present invention, a vehicle control system for distributing torque to primary and secondary drive trains is provided having a planetary gear set. The planetary gear set is made up of a sun gear, a ring gear, at least one planetary gear intermeshed with the sun gear and ring gear, and a planetary carrier rotatably connected with the at least one planetary gear. The vehicle control system also comprises a torque sink connected with the sun gear. The vehicle control system further comprises a torque input shaft. The torque input shaft is operably connected with the planetary carrier and is also coupled with a primary drive train. The vehicle control system also comprises a controller and sensors in communication with the controller wherein the sensors monitor vehicle conditions. When the torque sink applies a braking force to the rotation of the sun gear in response to instructions from the controller, the ring gear outputs torque to a secondary drive train.
In accordance with yet another aspect of the invention, a method for distributing torque between a primary drive train and secondary drive train is provided. In the first step a transfer case system is provided comprising a sun gear, a ring gear, at least one planetary gear intermeshed with the sun gear and ring gear, a planetary carrier rotatably connected with the at least one planetary gear, a torque sink connected with the sun gear, and an input shaft operably connected with the planetary carrier. In the next step, sensors and a controller monitor vehicle conditions for predetermined vehicle conditions that require torque transfer from a primary drive train to a secondary drive train. The torque sink is then instructed to apply a braking force to the rotation of the sun gear when the predetermined vehicle conditions are experienced. The ring gear then outputs torque wherein the ring gear applies torque to a secondary drive train. Finally, the braking force on the sun gear is removed when the predetermined vehicle conditions are no longer experienced.
Advantages of the present invention will become more apparent to those skilled in the art from the following description of the preferred embodiments of the invention which have been shown and described by way of illustration. As will be realized, the invention is capable of other and different embodiments, and its details are capable of modification in various respects. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.